The present invention relates to a device for extending thermocouple wires. More specifically, this device is used to minimize any error caused by ambient temperature fluctuations of a pair of connections points between a thermocouple and its corresponding extension wire pair.
Thermocouples have been recognized as temperature measurement devices since the early 1800's. They consist of two dissimilar metals joined at a temperature sensing junction. At this junction, a rise in temperature creates an EMF (electromotive force) within the circuit. When voltage is measured across an open circuit, it is a function of the junction temperature and the composition of the two dissimilar metals. If thermocouple wires need to be extended by use of an extension wire, an additional junction is added to the circuit and must be compensated for during voltage measurement. Many compensation techniques address the thermocouple to measurement circuit temperature variations, but they ignore the junction effect of the thermocouple to extension wire junction error. Much published literature leads us to believe that if the appropriate composition extension wire is used, then applications which use extension wire between actual thermocouple wire and cold junction compensation circuits will allow the measurement circuit to accurately measure hot junction temperatures. Test data indicates that applications where the thermocouple to extension wire connection point pairs fluctuate in temperature (25.degree. C. to 125.degree. C.), then significant temperature errors will be experienced by the measurement circuit. These errors arise from the additional thermocouple junction created by the thermocouple wire to extension wire connection.
The use of extension wire for extended length connections of noble metal thermocouples was introduced for two reasons--cost and maintainability. Since the cost of noble metal is high, the user in many applications will try to minimize the length and wire size of the noble metal thermocouple material. Although very small diameter wire (0.005 dia.) is adequate for many applications, the installation, handling, and maintenance of these wires is often impractical. A common solution is to use a large diameter thermocouple (0.020 dia.) with short leads (6 to 10 inches) to get the generated electromotive force to a more isothermal area, then to use a specifically formulated alloy wire to connect that junction to the measurement instrumentation.
One specific application where this technique is used is in the hot wall processes of semiconductor manufacturing. These process chambers operate in the 300.degree. C. to 1250.degree. C. range. It is common practice to use 6" to 8" type R or S (platinum-rhodium) thermocouples of #24 AWG wire. This allows for a rugged hot junction to be inside a heater wall while the wire ends are outside the heater wall in a more isothermal region. Because of the thermal losses of the heaters used in the manufacturing process, this more isothermal region will still often experience thermal fluctuations of as much as 70.degree. C. resulting in ambient temperature as high as 95.degree. C. The significance of this error is application specific and must be evaluated in terms of the total temperature error budget of the specific manufacturing process. If this additional error is significant relative to the thermal stability of the process, then an alternative solution should be considered.